Surface-enhanced Raman scattering (SERS) is a characteristic feature that is specifically observed in organic molecules adsorbed onto the surface of metal.
Specifically, SERS is a phenomenon in which metal particles come close to each other to generate a resonance effect on the surface of the metal particles, thereby inducing local surface plasmon, and then the local surface plasmon induces an enhanced electric field, which enhances Raman scattering from Raman active molecules (organic pigment molecules such as rhodamine 6G) adsorbed to the metal particles in the enhanced electric field.
One of the reasons why SERS draws attention is that a vibration spectrum can be obtained even from a single molecule or a single particle. This enables the detection of a trace amount of a chemical substance for biomolecular recognition, for example, and various studies have been actively carried out (Non-Patent Documents 1, 2, and 3).
Raman scattering is a physical phenomenon absolutely unsuitable for such trace detection originally due to some of its characteristics. The reason for this is considered as below.
Raman scattering is caused by a collision of a molecule to be measured with a photon emitted from laser light. For example, the number of photons emitted from an argon laser of a wavelength of 488 nm can be estimated to be about 2.5×1018 per second under an output power of 1 W. Among them, the number of photons coming into collision is only about 1013 to 1015, whereas most of the photons pass through without colliding with molecules. Such a slight number of photons come into collision in two collision modes of elastic collision and inelastic collision. In elastic collision, energy is not transferred between molecules and photons and scattering caused in this collision mode is called “Rayleigh scattering.” In Rayleigh scattering, energy is not transferred between photons and molecules, and hence the frequency of scattering light is accordingly the same as the wavelength of incident light. Most of the collisions between molecules and photons, which will occur at a low frequency as described above, are elastic scattering, and the scattering light is therefore largely Rayleigh scattering.
In inelastic scattering, on the contrary to elastic scattering, a photon comes into collision with a molecule and transfers its energy to the molecule. On this account, the frequency of the scattering light differs from the frequency of the incident light in contrast to Rayleigh scattering. This scattering is called Raman scattering. In particular, when Raman scattering light has a larger frequency than that of incident light (when a photon obtains energy from a molecule), such scattering is called anti-Stokes Raman scattering. When a photon conversely gives energy to a molecule, such scattering is called Stokes Raman scattering. The number of photons causing such inelastic collision is about 10−7 of the total number of photons coming into collision. As described above, with respect to the number of incident photons, the number of photons coming into inelastic collision to cause Raman scattering is very small. This leads to low detection sensitivity. This is why Raman scattering was rarely used for analysis means.
In the early 1970s, however, a large number of studies on resonance Raman, for example, the measurement of a resonance Raman scattering spectrum of gaseous halogen molecules by W. Holzer et al. (Non-Patent Document 4), were started to be reported. With an increase in scattering intensity by resonance Raman scattering (the intensity increase by the resonance Raman effect is typically about 103 to 105 times), Raman scattering has drawn attention. Resonance Raman is an effect of remarkably increasing the intensity of a Raman band derived from a vibration of a chromophore part corresponding to the absorption band when excitation light having a wavelength overlapping the absorption band of a certain molecule is used to measure Raman scattering. This enabled the Raman spectrum measurement of a pigment having a concentration of only several micromoles.
Then, in 1977, research groups of P. P. Van Duyne et al. (Non-Patent Document 5) and J. A. Creighton et al. (Non-Patent Document 6) independently found surface-enhanced Raman scattering. Three years before that, another research group of Fleischmann et al. actually observed the phenomenon, but they seemed to fail to recognize an increase in the scattering cross section as with the resonance Raman effect.
SERS generally means a phenomenon in which the Raman scattering intensity of a certain molecule adsorbed onto the surface of a metal electrode, sol, crystal, deposition film, or semiconductor is greatly enhanced compared with that of the molecule present in a solution. This phenomenon however still involves many unclear mechanisms. SERS is observed, for example, on gold, silver, copper, platinum, and nickel. A known feature of SERS is that the enhancement effect is especially large on silver and gold. The typical physical properties of SERS show the following dependencies.
1) The surface roughness of metal makes any contribution to expression of SERS.
2) An SERS spectrum typically shows clear wavelength dependence.
3) An SERS intensity depends on the orientation of molecules adsorbed on the surface of metal and also depends on the distance from the surface of the metal.
Two mechanisms have been proposed for expression of SERS so far. One of them is a surface plasmon model. Under this model, a reflectance spectrum is regarded as the absorption of surface plasmon generated by the collision of excitation light with the surface of metal and SERS occurs by a coupling between the molecular vibration of the adsorbed molecule and the surface plasmon excitation. The other model is called a charge transfer model. Under this model, a reflectance spectrum is regarded as the absorption of a complex formed by the surface of metal and a molecule and SERS occurs by the resonance Raman effect due to the absorption. In either case, although the mechanism has not been elucidated so far, it has been revealed that in the surface-enhanced Raman scattering in a condition satisfying both the resonance Raman condition and the SERS condition as described above, the scattering intensity increases by about 1011 to 1014 times. This greatly expands the potential of single molecule spectroscopy. Due to the high sensitivity, SERS has come to be applied to qualitative microanalysis.
As previously developed nanoparticles to be applied to SERS, for example, a method of adsorbing a low molecular aromatic ring compound onto the surface of a nanoparticle through direct electrostatic interaction and nanoparticles having a surface adsorbing a reporter molecule such as rhodamine, naphthalene, and quinoline having a thiol terminal (Non-Patent Document 7) have been applied as a molecular recognition sensor and a pH sensor (Non-Patent Document 8).
Examples of the known methods for synthesizing SERS particles include a method of synthesizing nano flake-like metal composite material (see Patent Document 1), a method of adsorbing a pigment (Raman active molecules) such as rhodamine 6G onto the surface of nano-porous material (see Patent Document 2), and a method using gold nanoparticles in which gold nanorods are immobilized onto a substrate and enhanced Raman scattering of the molecules on the surface is used for analysis (see Patent Document 3).
In addition, a plurality of synthesized organic-inorganic nanoclusters including a plurality of fused or aggregated metal particles that forms a metal cluster containing a plurality of Raman active organic compounds adsorbed onto the surfaces of a plurality of aggregated particles and to a plurality of junctions with which a plurality of metal particles firstly comes into contact is disclosed (see Patent Document 4).